Enhancement of efficiency in luminescent polymer by incorporation of conjugated 1,3,4-oxadiazole side chains as hole-blocker/electron-transporter

Abstract

A novel luminescent polymer poly(2-methoxy-5-{6′-[2″-(4‴-oxyphenyl)-5″-phenyl-1″,3″,4″-oxadiazole]-hexyloxy}-1,4-phenylenevinylene- alt-2,5-bis-dodecyloxy-1,4-phenylenevinylene) (MPPOXA), was synthesized by the Wittig reaction. Electron withdrawing pendant, 2-(4-oxyphenyl)-5-phenyl-1,3,4-oxadiazole (OXD), is separated from the main chain via linear 1,6-hexamethylene-dioxy chain. The band gap figured out from the UV-Vis spectrum and photoluminescence (PL) maximum of the polymer are 2.08 eV and 585 nm, respectively. These values are similar to those of MEH-PPV (2.12 eV and 580 nm). The maximum of electroluminescence (EL) of the device based on single layer structure (ITO/MPPOXA/Al) appeared at 586 nm, which is similar to that of MEH-PPV (583 nm). In PL and EL spectra, emission from OXD pendants was not observed. Single layer EL device based on MPPOXA have an external quantum efficiency of 0.01% at 2.3 mA/mm 2, which is significantly higher than that of MEH-PPV (0.0002% at 2.4 mA/mm 2) measured under the same conditions. The HOMO and LUMO energy levels of the polymer main chain figured out from the cyclic voltammogram and the UV-Vis spectrum are −4.96 and −2.88 eV, respectively, which are similar to those of MEH-PPV (−4.98, −2.86 eV). The estimated HOMO and LUMO energy levels of the pendant were −6.17 and −2.47 eV, respectively. LUMO energy level is significant lower than those of the main chain. These results suggest that OXD units do not affect the emission maximum of the main chain comparison with MEH-PPV. The pendants block the injected holes from the anode and enhance electron-transporting property.